Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Method Or preparing an alkali metal dirfusion
preventive layer
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention concerns a method of preparing
an alkali metal difrusion-preventive layer comprising a
silicate glass layer at the inside Or a substrate such as
a Si substrate and silicate glass substrate by using an ion
implantation method and, more in particular, it relates to a
method of preparing a phosphor silicate glass layer having
a performance for preventing high alkali metal difrusion.
Description o~ the Prior Art
Inorganic glass with improved surfaee property by
implantation of ioni~ed phosphorus from the surface has been
known in, for example, Japanese Patent Laid-Open Sho
56-84344 and Sho 56-116034. Further, as a method Or
manufacturing glass plates ~or producing semiconductor
device, there has been known a method Or ion implantaion of
phosphorus to the surface of a glass plate made Or an
alkali metal containing silicate glass, applying heat
treatment, further implantation of nitrogen ions and then
applying heat treatment again by, for example, Japanese
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Patent Laid-Open Sho 63-222046.
The method of ion implantation of phosphorus to the
sur~ace of the glass plate and applying heat treatment as
described above has a merit of forming an apparent phosphor
silicate glass layer at the inside of the glass plate
thereby enabling to improve the surface property of the
glass plate, but the effect of preventing the diffusion o~
alkali metal to the surface Or the glass substrate (so-
called gettering effect of the alkali metal) can not be
attained as expected and there has been a problem that
the gettering site is not efficiently formed on account of
insufficient oxidation of implantated phosphorus.
OBJECT AND SUMMARY OF THE INVENTION
The ob~ect of the present invention is to overcome
the foregoing problems and provide a method of preparing
an alkali metal diffusion-preventive layer capable of
manufacturing an alkali metal di~fusion-preventive layer
having higher performance of preventing alkali metal
dif~usion than that of alkali metal diffusion preventive
layer, manufactured in the conventional method.
In accordance with the present invention, one of
the following means (a) - (c) is adopted in a method of
implantation of phosphorus into a substrate containing silcon
thereby forming an alkali metal di~fusion-preventive layer
20~75B
containing phosphorus at the inside o~ the substrate:
(a) Ion implantation of phosphorus and ion implantation of
oxygen are applied simultaneously or alternately and,
subsequently, heat treatment is applied to the substrate.
(b) Ion implantation of phosphorus is applied and,
subsequently, ion implantation of oxygen is applied while
heating the substrate.
(c) Ion implantation of phosphorus is applied and, subse-
quently, heat treatment is applied to the substrate in
an atmosphere containing an oxygen gas.
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DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The heat treatment (a) described above is applied ~or
combining phosphorus and oxygen implanted to the inside of
the substrate with silicon in the ~ubstrate to each other,
thereby stabilizing the glass structure, which is preferably
applied at a temperature higher than 400 C. If the tempe-
rature is lower than 400 C, less stabiliz~tion occurs and,
~accordingly, the effect of the present invention is less
developed. On the other hand, if the temperature is higher
than the distortion point of glass, it tends to cause
deformation of the substrate, diffusion of the phosphor
silicate glass layer, etc. The heat treatment a~ter the
ion implantation of oxygen may be conducted in an oxygen gas-
containing atmosphere or in an inert gas atmosphere. In
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particular, if the substra~e is made of such material as
Si that denatures by the heat treatment in the oxygen gas
atmosphere, use of the inert gas atmosphere is desirable
since the surface oxidation ean be prevented. Phosphorus
and oxygen may be implanted repeatedly.
Heating of the substrate upon ion implantation of oxygen
in (b) above is a procedure required for stabilizing to
attain the glass structure in the same meanings as the
heat treatment described above and, preferably, conducted
at a temperature higher than 400 C. If the temperature is
lower than 400 C, the effect of the present invention is
less developed as described above. Further, if the tempe-
rature is higher than the distortion point of glass, it
tends to cause remarkable deformation of the substrate or
diffusion of phosphorus in the implanted state.
In the means (a) and (b), the acceleration energy and
the amount or phosphorus and oxygen can properly
be controlled as necessary depending on the material o~
the substrate, the amount of alkali metal impurity in the
substrate, required alkali metal diffusion preventive
performarlce, etc., and it is usually preferred to set the
acceleration energy to 1 KeV - 5 MeV and the dose
to 1 x 1012 to 1 x 1018/cm2. Further, it is preferred for
the implantation of phosphorus and oxyyen ions such that are
dispersed substantially in the same depth at the inside of
the substrate.
The heat treatment in the oxygen atmosphere in (c)
above is applied for di~Pusing oxygen in the atmosphere
Prom the surface to the inside of the substrate and
combining with phosphorus present at the inside of the
substrate by the ion implantation, and the temperature, time,
etc. for the heat treatment are controlled depending on
the acceleration energy or dose of phosphorus,
material for the substrate and oxygen concentration in the
atmospheric gas, etc. Higher oxygen concentration in the
atmosphere is preferred since the processing time can be
shortened. Further, it is preferred to increase the heat
processing temperature as high as possible within a range
of restriction caused by the material Por the substrate
(melting point, softening point, etc.) since the processing
time can be shortened with higher processing temperature.
A required time t for the heat processing can be
roughly determined as : t 2 L2/DoX in which L represents
the maximum in~ection depth (m) of phosphorus and DOx(m2/s)
represents the diffusion coefficient of oxygen at the heat
treating temperature.
The substrates usable herein are those containing Si,
at the surface, silicon, siicon nitride, quartz, quartz
glass, non-alkali glass and alkali-containing glass. In
particular, alkali-containing glass such as soda lime
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glass, low alkali content glass such as borosili~ate glass
are preferred since they show good productivity and pre-
ferred durability.
The present invention is based on the knowledge that
the gettering effect of the alkali metal in the conventio-
nal phosphor silicate glass layer formed at the inside o~
the substrate ~y the ion implantation of phosphorus can not
be attained as expected, because oxygen in the phosphor
silicate glass layer is insufficient to provide a stable
state and "negatively polarized non-bridging oxygen"
for capturing alkali metal is less Pormed. According
to the present invention, oxygen is supplied into the
pho~phor silicate glass by ion implantation of oxygen or
diffusion of oxygen, by which negatively polarized non-
bridging oxygen due to the presence of phosphor is
formed enough to provide a sufficient gettering effect
of the alkali metal.
In particular, by the method of supplying oxygen by
the ion implantation of oxygen, oxygen can be supplied in a
required amount only at a required depth.
Further, by the ion implantation of oxygen, since alkali
metal present from the outermost surface of the substrate
to the phosphor silicate glass layer is moved along the
direction of depth by the ion implantation and the alkali
metal content in that portion can be reduced, the alkali
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metal diffusion-preventive performance can ~urther be
improved.
Examples and Comparative Examples
Alkali-alumina-sili~ate glass containing 0.1 to 3
atom% o~ alkali metal was used as a substrate.
- At first, phosphorus was ion implanted by 1 X 1017/cm2
and at an acceleration energy at 140 KeV. The specimen
is herein referred to as A. The specimen A was applied
with heat treatment in a nitrogen atmosphere at 600 C for
one hour to prepare a specimen B. Further, oxygen was ion
implanted by 2.5 X 1017/cm2 under an acceleration energy at
70 KeV to the specimen A. The heat treatment was applied
at a temperature of 600 C ror one hour. There was no
particular restriction ~or the atmosphere. The specimen
is referred to as the specimen C.
Further, the specimen A was applied with heat treat-
ment at 600 C for one hour in an oxygen atmosphere to
~ prepare a specLmen D (although there was no data for the
;~ diPrusion coe~ficient o~ oxygen at 60o-c in this glass
1/2
composition, ~ ~ 6 ~m/h for the silica glass is referred
to). The diffusion length of oxygen is up to 6 ~m, which
is suf~iciently longer than the average projection flying
distance 143 nm of P implanted at an acceleration energy of 140
KeV. When the specimens B, C and D were measured by SIMS,
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the Na concentration near the surrace was increased in the
order of: C D B and the sur~ace Na concentration to the
bulk Na concentration was restricted to about 1/10,000 for
C, about 1/5,000 for D and about 1/1,000,000 for B. A
polycrystalline silicon thin ~ilm transistor (hereinafter
simply referred to as TFT) was formed to each of the
substrates B, C and D and a threshold voltage for turning
TFT to ON and aging stability was evaluated. As a result,
there was recognized no difference for the characteristics
o~ the TFT formed on the substrates C, D to those prepared
on quartz glass. However, for the TFT formed on the
substrate B, the threshold voltage was increased by up to
50Z than that formed on the quartz glass substrate and the
drain current after 1000 sec was reduced by 20~ than that
of the TFT formed on a quartz glass substrate.
According to the present invention, as apparent from
examples, it is possible to prepare an alkali metal dif-
fusion-prevent$ve layer of higher performance for preven-
ting alkali metal diffusion than that of the alkali metal
diffusion preventive layer prepared by the conventional
method.
The present invention is useful, for example, as a
method of modifying inexpensive alkali-containing glass
substrates into a substrate for electronic devices, or a
method of preventing undesired effects of alkali metals as
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much as possible in a substrate for electronic devicesmade of metal containing a trace amount of alkali metal
as impurity.
Having described a specific preferred embodiment of
the present invention, it is to be understood that the
invention is not limited to that precise embodiment, and
that various changes and modifi~ations may be effected
therein by one skilled in the art without departing from
the scope or spirit of the invention as defined in the
appended claims.
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